1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus and to a carrier device equipable with shim elements.
2. Description of the Prior Art
Magnetic resonance technology is a known technology for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields are generated by a gradient coil system and are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the resulting magnetic resonance signals, on the basis of which magnetic resonance images are produced.
A high homogeneity of the basic magnetic field is a critical factor for the quality of the magnetic resonance images. Inhomogeneities of the basic magnetic field within a homogeneity volume of the magnetic resonance apparatus cause geometrical distortions of the magnetic resonance image that are proportional to the inhomogeneities. Shim systems are employed for improving the basic magnetic field homogeneity within the homogeneity volume. A distinction is made between passive and active shim systems. In a passive shim system, a number of laminae composed of a magnetic material, particularly a ferromagnetic iron alloy, are attached in the examination space of the magnetic resonance apparatus in a suitable arrangement. To that end, the basic magnetic field is measured within the homogeneity volume before the attachment of the laminae. Using the measured values, a computer program determines the suitable number and arrangement of the laminae.
U.S. Pat. No. 5,786,695 discloses an essentially hollow-cylindrical magnetic resonance apparatus having an essentially hollow-cylindrical gradient coil system. Between its gradient coils and the appertaining shielding coils, the gradient coil system has an essentially hollow-cylindrical receptacle space for carrier devices that are equipable with shim elements. The carrier devices roughly correspond in length to the longitudinal extent of the gradient coil system and are secured to a part of the gradient coil system at the side of the shielding coil. The carrier devices uniformly distributed in circumferential direction of the gradient coil system. Since a constant temperature of the shim elements is important for maintaining a constant precision of the shim effect, the carrier devices are designed in the following way: Toward that the shielding coil side of the gradient coil system, the carrier devices have two flow-through webs in the longitudinal direction so that cylindrical cavities extending in the longitudinal direction are formed limited by the webs, the part of the gradient coil system at the side of the shielding coil and the actual carrier device. Air can be blown through these cavities for cooling purposes. Among other things, heat generated in the gradient coil system can be carried off effectively from the shim elements.